This study will evaluate the efficiency of removal of selected probe drugs from patients with end stage renal disease (ESRD) who are receiving continuous veno-venous renal replacement therapy by: (1)assessing the effect of blood flow rate on drug clearance, (2)assessing the effect of dialysate flow rate on drug clearance, (3)determining if drug removal is dependent on filter membrane composition, and (4)ascertaining the effect of protein binding on drug removal during diffusion and ultrafiltration.